A third youth surrendered Wednesday in the "spur-of-the-moment" sniper attack that killed a man and wounded his wife and their guide during a rafting tour on the Rio Grande, and the search continued for a fourth suspect.

The suspect, identified only as a minor, surrendered to Mexican authorities at Ojinaga in Chihuahua state, said Hugh Rushton, chief U.S. Border Patrol agent in Marfa.Law enforcement agencies on both sides of the border pressed a two-nation manhunt for a fourth suspect, whose identity was known to investigators.

Presidio County Sheriff Rick Thompson said there apparently was no motive for the attack. "From everything we have been able to determine, it was a spur-of-the-moment thrill killing," he said.

Border Patrol agents arrested the first two suspects on Tuesday.

Eduardo Pineda Rodriguez, 17, a legal resident alien, was arrested at his home in Redford, about 25 miles east of Presidio and was being held on a murder charge in $50,000 bond.